A telecentric projection lens, which is necessary especially for a projector to enhance a reflection efficiency of a display device, is configured to locate its entrance pupil significantly apart from the display device toward a screen for the purpose of enhancement of the reflection efficiency. The display device using such a telecentric zoom lens includes a combination of a liquid crystal device, digital micromirror device (DMD), and a TIR prism, and we recently have found drastic needs of the DMD in the market. In addition, for recent years, commercially available has been a rear projection TV set in which the telecentric zoom lens is incorporated.
Under these circumstances, the requirements of the telecentric zoom lens currently needed in the industry are as follows:
(1) Higher brightness (i.e., smaller numerical aperture). This is useful to project brighter beams on the screen. In order to direct the light flux from the light source as much reduced loss as possible to the screen, the projection lens must be increased in its pupil diameter so as to reduce the numerical aperture.
(2) Wider angle of view. Even in a relatively narrow residential space in Asia and Europe, it is desired that a sufficiently large image should be projected at a reduced distance to the screen.
(3) Reduced distortion. Desirably, a picture projected on the screen must not be distorted, and viewers seeing the distorted picture on the screen would really invoke some prompt emotional response of not feeling good.
(4) High resolution and reduced chromatic aberration. It is significant for either the liquid crystal device or the DMD to make pixel data reproducible with fidelity as complete as possible, and the required resolution is to be a performance of enabling the device to sufficiently resolve the pixels. However, fully resolving the pixels does not necessarily bring about attainments of sufficient resolution, picture quality, and color reproducibility after all due to color drift. Thus, it is necessary to minimize the chromatic aberration by means of low dispersion glass, but still required are additional particular concerns about confirming the correlation of the intended performance in design to the actually eye-inspected picture quality and refining the design in consideration of the predictable cost.
(5) Smaller diameters of lenses. This is especially related to a specification of outer dimensions of a main projector, and the downsizing or portability is an inevitable requirement. The thicknesswise dimension of the projector straightforwardly depends upon a diameter of a projection lens.
(6) Holding down on a cost increase due to featuring the zooming. The zooming function (power varying optics) has been prevailing as being essential as a user friendly feature that enables the operator to vary a projection size more easily. As is often the case, however, the feature of the zooming adds a more complicated mechanical drive system to the projector, and instead, the optical and mechanical designs must be made deliberating on avoiding the cost increase.
Among the telecentric zoom lenses for currently commercially available projectors, none of them meet all the above-mentioned requirements (1) to (6). Some sacrifices the requirements (1), (2) and (5) for the remainings (3) and (4), for example.
Some prior art zoom lens dedicated to the projector has a four-lens-group structure that typically consists of a first group of lenses adjustably moved forward for the focusing, a second group of lenses moved for the power varying, a third group of lenses serving as a compensator, and a fourth group of lenses fixed and stationary (see Patent Document 1 listed below).
Configured in this way, it is hard to unify the imaging ability relative to the varied distance to the screen especially because only the first group of lenses are adjusted for the focusing, and the resultant minimum projection distance to the screen tends to be even longer. In order to make this distance conveniently fall in a close-up range with a certain wide angle of view, an effective aperture of the foremost lens is liable to be greater. A cam driving the third group of lenses often provides a kinematic flex point(s), and this is an issue of machineability of the cam.
Moreover, the aforementioned prior art lens has an aspherical lens of spherical surface complex type which gives not so large an area of the aspherical surface and is insufficient to compensate for the distortion. This is why it is particularly hard to widen the angle of view. If the aspherical lens took a large area of the aspherical surface, its thickness should vary considerably from the center to the periphery, and some design tends to highly strengthen the lens power, which is resultantly prone to cause the lens to greatly alter its performance relative to a temperature variation when the lens is a typical lens of resin.
Furthermore, in the optics where the foremost lens has its foremost surface contoured in aspherical plane, the configuration is effective simply in compensating for the aberration but not advantageous in the user's handling in that he or she should touch the foremost surface or otherwise is likely to get it scratched.
Another prior art embodiment of the projection lens has an arrangement of three groups of lenses G1 to G3. The first group of lenses G1 consists of a first lens L1 positive in refractive power with a conjugate surface oriented to a major conjugate side, a second lens L2 negative in refractive power with a sharp concave surface oriented to a minor conjugate side, and a third lens L3 negative in refractive power with a sharp convex surface oriented to the minor conjugate side. The second group of lenses G2 includes a fourth lens L4 positive in refractive power, and an aperture stop AST is provided in the vicinity of the minor conjugate side and close to a focal point on the major conjugate side for the third group of lenses G3. The third group of lenses G3 consists of a fifth lens L5 negative in refractive power with a concave surface oriented to the minor conjugate side, a sixth lens L6 positive in refractive power with a convex surface oriented to the major conjugate side and joined to the fifth lens L5 thereon, and seventh and eighth lenses L7 and L8 respectively positive in refractive power (see Patent Document 1 listed below).
This projection lens is a monofocal lens and is dedicated to a rear projector, and when it is used in a front projector, a focal length becomes shorter to experience difficulty in focusing by adjusting the optics. The lens dedicated to the rear projector should have the first lens got greater in diameter, and this impedes the downsizing of the projector.
Another prior art embodiments of the projection lens are shown in Patent Documents 2 to 5, and none of them meet all the above-mentioned requirements (1) to (6).